Re: [PATCH net] net: flower: fix stack-out-of-bounds in fl_set_key_cfm()

[Ido Schimmel <idosch@idosch.org>]

On Tue, Jul 25, 2023 at 08:44:42AM +0200, Zahari Doychev wrote:
> On Mon, Jul 24, 2023 at 04:32:54PM +0000, Eric Dumazet wrote:
> > Typical misuse of
> > 
> > 	nla_parse_nested(array, XXX_MAX, ...);
> > 
> > array must be declared as
> > 
> > 	struct nlattr *array[XXX_MAX + 1];
> > 
> > Fixes: 7cfffd5fed3e ("net: flower: add support for matching cfm fields")
> > Reported-by: syzbot <syzkaller@googlegroups.com>
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Cc: Simon Horman <simon.horman@corigine.com>
> > Cc: Ido Schimmel <idosch@nvidia.com>
> > ---
> >  net/sched/cls_flower.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
> > index 8da9d039d964ea417700a2f59ad95a9ce52f5eab..3c7a272bf7c7cf7d4ae21b5370cbc428086d6979 100644
> > --- a/net/sched/cls_flower.c
> > +++ b/net/sched/cls_flower.c
> > @@ -1709,7 +1709,7 @@ static int fl_set_key_cfm(struct nlattr **tb,
> >  			  struct fl_flow_key *mask,
> >  			  struct netlink_ext_ack *extack)
> >  {
> > -	struct nlattr *nla_cfm_opt[TCA_FLOWER_KEY_CFM_OPT_MAX];
> > +	struct nlattr *nla_cfm_opt[TCA_FLOWER_KEY_CFM_OPT_MAX + 1];
> 
> I think we need to redefine TCA_FLOWER_KEY_CFM_OPT_MAX like this as well:
> 
> diff --git a/include/uapi/linux/pkt_cls.h b/include/uapi/linux/pkt_cls.h
> index 7865f5a9885b..4f3932bb712d 100644
> --- a/include/uapi/linux/pkt_cls.h
> +++ b/include/uapi/linux/pkt_cls.h
> @@ -710,9 +710,11 @@ enum {
>         TCA_FLOWER_KEY_CFM_OPT_UNSPEC,
>         TCA_FLOWER_KEY_CFM_MD_LEVEL,
>         TCA_FLOWER_KEY_CFM_OPCODE,
> -       TCA_FLOWER_KEY_CFM_OPT_MAX,
> +       __TCA_FLOWER_KEY_CFM_OPT_MAX,
>  };
>  
> +#define TCA_FLOWER_KEY_CFM_OPT_MAX (__TCA_FLOWER_KEY_CFM_OPT_MAX - 1)

Yes, I believe you are right. "MAX" should be the value of the highest
valid attribute. That's not the case with "TCA_FLOWER_KEY_CFM_OPT_MAX".
Need to adjust "cfm_opt_policy" as well.

Tested [1] and it works fine:

# cd tools/testing/selftests/net/forwarding/
# ./tc_flower_cfm.sh 
TEST: CFM opcode match test                                         [ OK ]
TEST: CFM level match test                                          [ OK ]
TEST: CFM opcode and level match test                               [ OK ]

[1]
diff --git a/include/uapi/linux/pkt_cls.h b/include/uapi/linux/pkt_cls.h
index 7865f5a9885b..4f3932bb712d 100644
--- a/include/uapi/linux/pkt_cls.h
+++ b/include/uapi/linux/pkt_cls.h
@@ -710,9 +710,11 @@ enum {
        TCA_FLOWER_KEY_CFM_OPT_UNSPEC,
        TCA_FLOWER_KEY_CFM_MD_LEVEL,
        TCA_FLOWER_KEY_CFM_OPCODE,
-       TCA_FLOWER_KEY_CFM_OPT_MAX,
+       __TCA_FLOWER_KEY_CFM_OPT_MAX,
 };
 
+#define TCA_FLOWER_KEY_CFM_OPT_MAX (__TCA_FLOWER_KEY_CFM_OPT_MAX - 1)
+
 #define TCA_FLOWER_MASK_FLAGS_RANGE    (1 << 0) /* Range-based match */
 
 /* Match-all classifier */
diff --git a/net/sched/cls_flower.c b/net/sched/cls_flower.c
index 8da9d039d964..9f0711da9c95 100644
--- a/net/sched/cls_flower.c
+++ b/net/sched/cls_flower.c
@@ -776,7 +776,8 @@ mpls_stack_entry_policy[TCA_FLOWER_KEY_MPLS_OPT_LSE_MAX + 1] = {
        [TCA_FLOWER_KEY_MPLS_OPT_LSE_LABEL]    = { .type = NLA_U32 },
 };
 
-static const struct nla_policy cfm_opt_policy[TCA_FLOWER_KEY_CFM_OPT_MAX] = {
+static const struct nla_policy
+cfm_opt_policy[TCA_FLOWER_KEY_CFM_OPT_MAX + 1] = {
        [TCA_FLOWER_KEY_CFM_MD_LEVEL]   = NLA_POLICY_MAX(NLA_U8,
                                                FLOW_DIS_CFM_MDL_MAX),
        [TCA_FLOWER_KEY_CFM_OPCODE]     = { .type = NLA_U8 },
@@ -1709,7 +1710,7 @@ static int fl_set_key_cfm(struct nlattr **tb,
                          struct fl_flow_key *mask,
                          struct netlink_ext_ack *extack)
 {
-       struct nlattr *nla_cfm_opt[TCA_FLOWER_KEY_CFM_OPT_MAX];
+       struct nlattr *nla_cfm_opt[TCA_FLOWER_KEY_CFM_OPT_MAX + 1];
        int err;
 
        if (!tb[TCA_FLOWER_KEY_CFM])